KVM: x86: Fix a semi theoretical bug in kvm_arch_async_page_present_queued()

Fix a semi theoretical race condition related to a lack of memory barriers
when dealing with vcpu->arch.apf.pageready_pending.  In theory, the "ready"
side could see a stale pageready_pending and neglect to kick the vCPU, and
thus allow the vCPU to enter the guest with a pending KVM_REQ_APF_READY
and no kick/IPI on the way, in which case the KVM would fail to deliver a
completed async #PF event to the guest in a timely manner as the request
would be recognized only on the next (coincidental) VM-Exit.

kvm_arch_async_page_present_queued() running in workqueue context:

    kvm_make_request(KVM_REQ_APF_READY, vcpu);
    /* memory barrier is missing here*/
    if (!vcpu->arch.apf.pageready_pending)
        kvm_vcpu_kick(vcpu);

kvm_set_msr_common() running in task context:

    vcpu->arch.apf.pageready_pending = false;
    /* memory barrier is missing here*/

And later, vcpu_enter_guest() running in task context:

    if (kvm_check_request(KVM_REQ_APF_READY, vcpu))
         kvm_check_async_pf_completion(vcpu)

Add missing full memory barriers in both cases to avoid theoretical
case of not kicking the vCPU thread.

Note that the bug is mostly theoretical because kvm_make_request()
uses an atomic operation, which is always serializing on x86, requiring
only for documentation purposes the smp_mb__after_atomic() after it
(smp_mb__after_atomic() is a NOP on x86).

The second missing barrier, between kvm_set_msr_common() and
vcpu_enter_guest(), isn't strictly needed because KVM executes several
barriers in between calling these functions, however it still makes
sense to have an explicit barrier to be on the safe side and to document
the ordering dependencies.

Finally, also use READ_ONCE/WRITE_ONCE.

Thanks a lot to Paolo for the help with this patch.

Link: https://lore.kernel.org/all/7c7a5a75-a786-4a05-a836-4368582ca4c2@redhat.com
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://patch.msgid.link/20251015033258.50974-3-mlevitsk@redhat.com
[sean: explain the race and its impact in more detail]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: Maxim Levitsky

arch/x86/kvm/x86.c

@@ -4183,7 +4183,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		if (!guest_pv_has(vcpu, KVM_FEATURE_ASYNC_PF_INT))
 			return 1;
 		if (data & 0x1) {
-			vcpu->arch.apf.pageready_pending = false;
+			/*
+			 * Pairs with the smp_mb__after_atomic() in
+			 * kvm_arch_async_page_present_queued().
+			 */
+			smp_store_mb(vcpu->arch.apf.pageready_pending, false);
+
 			kvm_check_async_pf_completion(vcpu);
 		}
 		break;
@@ -13890,7 +13895,7 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
 	if ((work->wakeup_all || work->notpresent_injected) &&
 	    kvm_pv_async_pf_enabled(vcpu) &&
 	    !apf_put_user_ready(vcpu, work->arch.token)) {
-		vcpu->arch.apf.pageready_pending = true;
+		WRITE_ONCE(vcpu->arch.apf.pageready_pending, true);
 		kvm_apic_set_irq(vcpu, &irq, NULL);
 	}
 
@@ -13901,7 +13906,11 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
 void kvm_arch_async_page_present_queued(struct kvm_vcpu *vcpu)
 {
 	kvm_make_request(KVM_REQ_APF_READY, vcpu);
-	if (!vcpu->arch.apf.pageready_pending)
+
+	/* Pairs with smp_store_mb() in kvm_set_msr_common(). */
+	smp_mb__after_atomic();
+
+	if (!READ_ONCE(vcpu->arch.apf.pageready_pending))
 		kvm_vcpu_kick(vcpu);
 }